This application claims priority from Korean Patent Application No. 2002-74362, filed on Nov. 27, 2002, in the Korean Intellectual Property Office, the disclosure of which is incorporated herein in its entirety by reference.
1. Field of the Invention
The present invention relates to a solid electrolyte and a battery employing the solid electrolyte, and more particularly, to a solid electrolyte with improved ionic conductivity and electro-chemical stability and a lithium battery and a thin-film battery that employ the same.
2. Description of the Related Art
With rapid advance in computer technology and the mobile communications field, the development of digital, smaller, and multimedia information devices have been accelerated. Lithium batteries, which are lightweight and have high energy density, are taking the initiative in the market as an energy source of typical portable information devices, such as notebook personal computers, personal digital assistants (PDAs), mobile phone terminals.
The size of batteries can be a limiting factor to the minimization of such electronic information devices. For example, the size of complementary metal oxide semiconductors (CMOS), smart integrated circuits (ICs), micro-sensors, micro-robots are limited by the size of their battery. Based on the advances in semiconductor manufacturing processes and micro-electro mechanical systems (MEMS), the use of thin-film batteries as an energy source for next-generation micro-systems is expected to increase.
FIG. 1 shows the structure of a conventional thin-film battery. Referring to FIG. 1, a conventional thin-film battery basically includes a cathode 12, an electrolyte 14, and an anode 13, which are sequentially deposited as films on a current collector 11. The conventional thin-film battery has a thickness of about 10 μm.
Since the anode 13 is arranged close to the cathode 12, the conventional thin-film battery has advantages of higher current density, higher battery efficiency, and shorter migration distance of ions. Since ions can migrate across the battery more easily and rapidly, the amount of reactants can be cut down greatly. Another advantage of the conventional thin-film battery is that it is easier to vary the shape and size of batteries for particular purposes. Therefore, thin-film batteries are considered to be the most promising main power source for micro-electronic devices, MEMS devices, micro-sensors.
Thin-film batteries are manufactured by the same method as semiconductor manufacturing processes. Accordingly, a thin-film battery may be mounted as a back-up power source along with electronic circuits in a semiconductor chip such as a complementary metal oxide semiconductor memory chip. In other words, the dead space of electronic devices can be minimized with maximum space utilization efficiency when a thin-film battery is incorporated. Various batteries with different voltage and capacitance can be implemented through proper designing and etching for serial and parallel connections, and thus they have wide applications.
Unlike conventional lithium ion batteries, thin-film batteries require a perfect solid Li+ conducting electrolyte. Electrolyte materials, such as LiSiCON, Li4SiO4—Li3PO4 solid solution, Li2O—B2O3—SiO2,Lipon ( lithium phosphorous oxynitride), etc. are considered to be suitable for thin-film batteries.
Although the above-listed crystalline solid electrolytes are known as having effective Li+ conductivity, inorganic compounds with complex crystalline structure are amorphous when deposited as a thin film and require high temperature thermal treatment for crystallization. Therefore, it is impractical to apply such inorganic compounds to manufacture thin-film batteries.
In contrast, glass electrolytes with high isotropic conductivity in amorphous state are easier to process into thin film form compared to crystalline electrolytes. The ionic conductivity of glass electrolytes varies depending on their composition, and the composition of thin film electrolytes can be easily adjusted during deposition. In addition, in thin-film batteries whose electrodes are spaced merely several micrometers apart, as low ionic conductivity as 10−7 S/cm is satisfactory for battery formation. Therefore, the comparatively low ionic conductivity of glass solid electrolytes is not important in thin-film batteries.
The currently most attractive solid electrolyte for thin-film batteries is Lipon, which is disclosed by John B. Bates et al. in U.S. Pat. No. 5,338,625, entitled “Thin film Battery and Method for making the Same.” The LiPON solid electrolyte, which is manufactured with a Li3PO4 target by radio frequency sputtering in a nitrogen atmosphere, has a high ionic conductivity of 2(±1)×10−6 S/cm. In addition, the Lipon solid electrolyte forms a very stable interface with an anode or a cathode and allows the battery to less deteriorate during operation. The Lipon solid electrolyte satisfies most requirements for use in thin-film batteries employing a solid electrolyte.
However, the properties of the Lipon thin film are greatly dependent on processing parameters for thin film formation and the Lipon thin film lacks reproducibility (P. Birke et al., Materials for thin film batteries for application in silicon technology, Solid State Ionics 93 (1997), 1–15). For this reason, considerable time is required to solve such above-described problems before Lipon thin films can be mass produced.
Therefore, for the development of thin-film batteries as a powerful energy source of 21st century leading micro-systems, it is essential to develop glass solid electrolytes with improved ionic conductivity.